The electrical conductivity of electrolytes in the majority of cases has been traditionally measured using a contact method. In such case the trustworthiness of the obtained results is greatly influenced by the contact phenomena. Let us discuss it on the basis of an example.
Let us assume that two identical metallic electrodes, for example, two platinum plates are immersed into a water solution of a copper vitriol. Let us pass current through the copper vitriol solution with the platinum electrodes by connecting the latter to an outer e.m.f. source. Then copper will be deposited on the platinum electrode that serves as a cathode while oxygen will be formed on the anode. This will result in a disturbance of the electrodes symmetry: one of them will be coated with a copper layer while the other will have a film of oxygen. This is also accompanied by a change in the electrolyte concentration at the electrodes: the salt concentration at the cathode decreases while an acid is formed at the anode. Now the electrodes immersed in the solution will not be identical, forming a galvanic element with its own e.m.f. E′, the so called polarization element [1].
A similar disturbance of the initial symmetry that exists when the electrodes are of an identical material, will occur each time during the electrolysis of solutions. The change of the electrodes that occurs in such case is termed as polarization of electrodes, while the generated e.m.f. is termed as polarization e.m.f. Such polarization is not always reversible: after the discharge of the element the initial state may not be restored.
The generation of the polarization e.m.f. creates a number of serious problems.
In order to be deposited on the electrode the ions have to overcome a certain potential barrier. Work A′=E′It is spent to overcome this barrier. The work spent for the emission of the Lenz-Joulean heat Q=I2Rt can be reduced; to this end it is necessary to change resistance R by using large electrodes to be arranged closely to one another. But work A′ is unavoidable, and its value is a function of the electrolyte composition and the material of the electrodes. Value E′ equals polarization e.m.f. only in cases when polarization of the electrodes is reversible. But in the majority of cases it is higher. In order to have the process of ions deposition on the electrode started, “overvoltage” is required which may equal several tenths of a Volt. Thus, in case of a reversible polarization of electrodes to start electrolysis of a sulfuric acid water solution, a 1.22 V difference of potentials would be required on the electrodes; but actually, when pure platinum electrodes are used, the electrolysis (that in this case leads to water decomposition) starts only at 1.64 V.
To weaken the influence of the electrochemical reactions on the electrolyte resistance being measured, the conductivity is measured using alternating current. This method is well known. The companies that produce liquid electrolytes indicate in their certificates the conductivity values measured at the alternating current, with the characteristics of the electrodes that are in contact with the electrolyte.
Among the disadvantages of the method for electrolyte conductivity measuring by using contact electrodes and the alternating current is the following. Depending on the nature of the solution and the mechanism of its ion conductivity in the general case for each solution there exists its own range of the alternating current frequency within which the conductivity does not depend on frequency. Therefore for each case it is necessary to determine this frequency range and the acceptable value of the alternating current voltage that is applied to the electrodes. Nevertheless this condition is not enough to guarantee the measurement reliability. The next important condition is the stability of the conductivity value being measured during a lengthy period. That will guarantee that on the electrodes within the given frequency range and given voltage there will be no reactions that may change the condition of the system.
The errors inherent to the contact methods of measuring the electric properties of electrolytes can be avoided by using noncontact eddy-current methods of measuring.
Known in the art are devices for measuring electrical conductivity of electrolytes using an eddy-current method.
According to U.S. Pat. No. 2,542,057, Relis, “Method and apparatus for measuring the conductivity of an electrolyte”, Feb. 20, 1951, the conductivity of a liquid electrolyte is determined by immersing therein insulated primary and secondary inductance coil of toroidal shape. The primary coil is excited by the alternating current, the field of this coil induces eddy current in the electrolyte. The eddy current field is fixed by the secondary col. The eddy current value is a function of the specific electrical conductance of the liquid electrolyte. A thin copper screen is used which is arranged above the coils. To reduce the voltage level not related to the signal, an external compensating circuit is used.
According to U.S. Pat. No. 3,806,798, Gross, “Electrodless conductivity measuring system”, Apr. 23, 1974, for measuring the conductivity of a liquid electrolyte an exciting and a measuring induction coils are also used According to this patent the interferences are compensated and the sensitivity of measurements are increased through relative arrangement of the coils, screening, introduction of an additional winding.
In U.S. Pat. No. 4,220,920, Gross, “Electrodless conductivity measuring system”, Sep. 2, 1980, the exciting and the measuring coils immersed into a conducting liquid whose electrical conductivity is measured according to the voltage induced by the eddy current in the liquid on the measuring coil, are arranged in a certain manner relative ot each other. According to this patent the metrological characteristics are upgraded due to the special shape of the screens of the coils, arrangement of the windings and the signal processing circuit.
The advantageous feature of these patents is the noncontact measurements of the electrical conductivity of liquid electrolytes.
A common disadvantage of these patents is that the measurements are performed at one fixed frequency, without considering the issues related to the frequency dispersion of the specific electrical conductance of liquid electrolytes.
In the patent U.S. Pat. No. 4,408,202, Fales, “Electrokinetic display system”, Oct. 4, 1983, is proposed a system for studying the electrical kinetics of solutions of liquid electrolytes with different concentrations. The electrolyte contains the reagents that take part in the reverse reaction during the electrolysis process. The reagents are self-induced or contain an indicator. Electric current is passed between the electrodes immersed into the liquid. To the electrolyte volume a variable magnetic field is applied, eddy currents are excited in the electrolyte while the magnetic field is spatially shifted. In such way the processes in the area of the electrodes are studied that are related to the movement and accumulation of the charge carriers in the electrolyte.
In the patent U.S. Pat. No. 4,820,990, Moore, “Electrodless detector”, Apr. 11, 1989, it is proposed to use an inductive primary transducer placed on the sampler in the form of a dielectric cylinder, for studying the position of the spatial charge (ion cloud) in the near-electrode area of the electric double layer in heterogeneous electrochemical liquids. The inductive transducer is excited at a fixed frequency. An electric circuit with a signal processor is used to process the signal depending on the conditions of forming a spatial charge in the liquid electrolyte.
An advantage of these patents is that the use of a vortex magnetic field allows sounding the position and movement of the charge carriers in the near-electrode areas without immersing into these electrolyte areas of additional contact electrodes that would cause changes in the position of the charges.
A disadvantage is that the proposed solutions do not allow determining the frequency characteristics of the process of charges transfer in the electrolyte.
In the patent U.S. Pat. No. 4,825,168, Ogava et al, “Remote conductivity sensor using square wave excitation”, Apr. 25, 1989, it is proposed to use an eddy current sensor in the form of an exciting and a pickup coils for on-line measuring the conductivity of a liquid. It is used to control the water quality in a water supply system. The sensor circuit includes an alternating current generator, digital timer, trigger, current-to-voltage converter, alternating current amplifier, synchronous detector.
An advantage of this patent is the noncontact measurement, its high sensitivity due to the use high frequencies of the field.
But when measuring the sensitivity of electrolytic media it is also necessary to take into regard the processes of periodic transfer (or spatial shift) of the linked charges under the influence of the alternating field that is not carried out according to this patent.
In the patent U.S. Pat. No. 5,157,332, Reese et al, “Three-toroid electrodless conductivity cell”, Oct. 20, 1992, it is proposed to use a three-coil system that is immersed into a liquid for measuring its electrical conductivity. The measurement is carried out in the stream of liquid. Two sensor configurations are used: in the first the two edge toroid coils are exciting while the middle one is pickup; in the second the middle coil is exciting while the two edge coils are pickup. The results obtained with the three-coil and two-coil systems are compared.
An advantage of this patent is the possibility of determining inhomogeneities within the volume of a liquid stream.
A disadvantage of the patent is the impossibility of studying conductivity within the frequency range.
In the patent FR 2782802 A1, Coudray et al, Mar. 03, 2000, parallel to the eddy current sensor a capacitor is switched in whose capacity can be varied. The parallel oscillatory circuit thus formed is switched to the generator of harmonic oscillations and the circuit is tuned in resonance with the generator frequency. The patent proposes using an oscillating circuit with a measuring circuit for measuring the specific electric resistance of an electroconducting liquid.
The use of a parallel resonant circuit offers advantages in the range of high frequencies (metric waves band) thus allowing to study high-resistance electrolytes.
But in the proposed circuit only one parameter of the eddy-current sensor can be measured: either the change of its active resistance or the change of its inductance at one frequency of the sounding field. There is no possibility for measurements within the frequency range.
The closest in its technical essence to the proposed invention is the method of noncontact measuring of the electrical conductivity of a liquid disclosed in the U.S. Pat. No. 5,077,525, West et al, “Electrodless conductivity sensor with inflatable surface”, Dec. 31, 1991. In the patent it is proposed to generate and excite an alternating magnetic field by means of an unductance coil of toroidal shape, to measure the eddy current field induced by the field of the exciting coil in the liquid using a pickup coil of the same shape. The voltage introduced into the pickup coil is a function of the eddy current intensity in the liquid that is determined at the fixed intensity value of the exciting magnetic field according to the conductivity of the liquid. The outer surface of the converter is separated from the liquid with a flexible nonconducting 2–3 mm thick membrane. The patent includes means that effect vibration of the membrane thus cleaning it off the deposit that is electroconductive. Such a deposit introduces a serious error into the results of measuring the electrical conductivity of a liquid electrolyte.
The main advantages of this patent noncontact measuring and means for self-cleaning of the membrane surface that separates the operating end face of the sensor from the liquid.
The disadvantages of this method are absence of means for measuring the frequency dependence of the electrolyte conductivity, and also the fact that the flexible membrane forms an intricate surface configuration of the liquid relative to the pickup inductance coil of the sensor. In view of this only an approximate evaluation of the specific electrical conductance of the liquid electrolyte is possible. This method can be effectively used only for comparing the relative electrical conductivity values of liquids having approximately equal density.